High density recording and playback techniques are currently used to achieve operational advantages and economies in data processing and storage systems. The current trend is toward increasingly high data storage densities. While the recording, storage and playback of high density data is advantageous, it has certain operational disadvantages. A primary one of these is the generation of erroneous signals when data is read from a high density medium. This is sometimes caused by a phenomenon termed intersymbol interference which occurs when bits read from a high density medium are not applied to the processing circuitry during their allotted sampling time. Instead, a data bit can appear either late (L) or early (E) rather than on-time (OT) relative to its bit cell clock window. This phenomenon is known as intersymbol interference or peak shifts. It occurs primarily in bipolar recording in which successive binary 1s are of the opposite polarity such as +1 -1 +1 or +1 .0.-1 .0..0.+1, etc.
A data bit shift can result in defective data when a binary 1 appears either early or later relative to its clock window. When it occurs, it corrupts the signal representing an adjacent binary .0.. Thus, a data pattern of +1 .0.-1 +1 may be corrupted by an early shift of the -1 so that the resulting data appears as +1 -1 -1 +1. A late data bit shift occurs when a data pattern such as -1 +1 .0.-1 is corrupted by a data bit shift of the +1 so that the resultant data pattern appears as -1 +1 +1 -1. A +1 or a -1 denotes that the amplitude of the respective data bit is above the detection threshold. A .0. denotes that the amplitude is below the detection threshold.
Heretofore, the problem associated with data bit shift or intersymbol interference has been minimized by a technique known as write pre-compensation. Since data bit shifts are typically associated with data patterns in which a binary 1 is adjacent one or more binary .0.s, this pattern can be detected in advance and the data bit shift problem may be avoided to some extent by the use of what is termed a write pre-compensation. The write pre-compensation functions by detecting the vulnerable data bit patterns and by altering the recording process (write pulses) so that the possibility of recorded data being corrupted by a data bit shift is minimized. The write pre-compensation performs this function by detecting the data bit patterns that are subject to either an early or late shift, and by moving the data bit that is vulnerable to a possible data bit shift in the direction required to minimize a possible corruption of an adjacent binary .0..
Although the use of write pre-compensation techniques tend to minimize, but not eliminate, the data bit shift problem, they have a number of disadvantages that preclude them from being the ultimate solution. First of all, it is not always possible to predict with complete accuracy the time by which a signal peak will shift with respect to its clock window. Therefore the write pre-compensation apparatus may either fail to alter the time at which a data bit subject to shift is recorded or, alternatively, may erroneously move a data bit that is not subject to data bit shift. Another problem with write pre-compensation is that the speed of the magnetic medium, such as a tape, may not be identical for both the recording and playback function and therefore, the data bit shift may not be correctly compensated for. Another factor is that playback apparatus that is designed to operate only with tapes that are written using the write pre-compensation techniques, will be subject to data bit shift errors if it plays back tapes that were recorded without the use of the write pre-compensation techniques.
It can be seen from the above that data bit shift problems can occur with high density recording and that the use of pre-qualifying sampled data write compensation techniques is only a partial solution to the problem. There is therefore a need for apparatus that compensates for the playback of shifted data bits so as to minimize the corruption of the data represented by the shifted bits.